According to the driving types for organic light emitting diode (mentioned as OLED hereafter) displays, the OLED displays are divided into passive matrix type OLED (mentioned as PMOLED hereafter) and active matrix type OLED (mentioned as AMOLED hereafter). The so-called active matrix type OLED (AMOLED) utilizes thin film transistors (mentioned as TFT hereafter) and capacitors to store signals and thereby controls the brightness and the gray scale of the OLED.
Although the manufacturing cost and the technical level of the passive matrix type OLED (PMOLED) are lower, the PMOLED is restricted by the driving method, and then the resolution of the OLED cannot be enlarged. Thus, the size of the product of the PMOLED is restricted within 5 inch, and the application of the product of the PMOLED will be restricted to the application of low resolution and small size. If the requirement of the product of the OLED is high resolution and large size, the main type is the active matrix type. The so-called active matrix type utilizes capacitors storing signals, so the pixel still maintains the original brightness after the line has been scanned. In contrast to the passive matrix type, the pixel cannot light until the scan line selects the pixel. Thus, it is not necessary that OLED of the active matrix type is driven to very high brightness, and the active matrix type OLED have a better lifetime and meet the requirement for high resolution. The OLED is integrated with the technology of the TFT to realize the active matrix OLED and the above-mentioned advantageous property of the OLED is fully expressed.
The manufacturing processes of the TFT formed on a glass substrate include amorphous silicon (mentioned as a-Si hereafter) manufacturing process and low temperature poly-silicon (mentioned as LTPS hereafter) manufacturing process. The differences between the LTPS TFT and the a-Si TFT are the electric characteristics of the TFT devices and the complexity of the manufacturing process. The mobility of the carrier of the LTPS TFT is higher than that of the a-Si TFT and the higher mobility of the carrier means that the TFT can provide much more electric current under the same voltage bias, but the manufacturing process of the LTPS TFT is more complex. Contrary to the LTPS TFT, the mobility of the carrier of a-Si TFT is less than that of LTPS TFT, but the manufacturing process of a-Si TFT is simple and superiorly competes with other ones in cost.
The manufacturing process of the LTPS is not mature, and the threshold voltage and the mobility of the LTPS TFT elements may vary, therefore, the property of each TFT element can be different. Although the same image data signal voltages are inputted to the pixels, the OLEDs of the pixels generate different output electric current, such that the brightness emitted by the OLED of the different pixel of a display panel is different. For above reason, the result leads the OLED display panel to display an image with erroneous gray scale and to have bad image uniformity.
In order to resolve the above-mentioned problem, U.S. Pat. No. 6,362,798, entitled “Transistor Circuit, Display Panel And Electronic Apparatus”, discloses a pixel circuit as shown in FIG. 5. The above-mentioned patent is characterized in that a compensating TFT M2 with a diode-connected type is disposed at the circuit between the terminal of a data signal voltage Vsig and a storage capacitor C2. An electric current flows from the data signal voltage terminal Vsig to a joint G, though a switching TFT M1, the compensating TFT M2 and the storage capacitor C2, and finally is equal to zero because the voltage of the joint G is higher and higher. Simultaneously, the compensating TFT has a voltage drop Vth_comp between two ends thereof, so the voltage of the joint G is equal to Vsig minus Vth_comp (Vsig−Vth_comp). Thus, the amount of an electric current I flowing through an OLED is equal to:I=(½)×β×(Vsg_driv−Vth_driv)2 I=(½)×β×(Vc−Vsig+Vth_comp−Vth_driv)2,wherein β is the transconductance parameter of the driving TFT M4. By the above-mentioned formulas, it is seen that if the Vth_comp is equal to a Vth_driv (the threshold voltage of the driving TFT M4) during the manufacturing process, the amount of output current of the OLED will not influenced by the threshold voltage of the driving TFT M4 and only depends on the amount of the data signal voltage Vsig. Thus, the driving TFT M4 which of the variance in the threshold voltage caused by the factor of the manufacturing process can be compensated.
In order to resolve similar conventional problem, a thesis, entitled “A New Modulated AMOLED Pixel Design Compensating Threshold Voltage Variation of Poly-Si TFTs”, published by Seoul University (Korea), also discloses a pixel circuit as shown in FIG. 6. The thesis is characterized in that a transistor P3 with a diode-connected type is disposed at the circuit between the terminal of a data signal voltage Vdata and a storage capacitor C3. An electric current flows from the terminal of the data signal voltage Vdata to the gate of a transistor P2, though a transistors P1, P3 and the storage capacitor C3, and finally is equal to zero because the voltage of the gate of the transistor P2 becomes higher and higher. Simultaneously, the transistor P3 has a voltage drop Vth3 between two ends thereof, so the voltage of the gate of the transistor P2 is Vdata minus Vth3 (i.e. Vdata−Vth3). Thus, the amount of an electric current I flowing through an OLED 650 is equal to:
 I=(½)×β×(Vsg2−Vth2)2I=(½)×β×(Vdd−Vdata+Vth3−Vth2)2,wherein β is the transconductance parameter of the transistor P2. The thesis being similar to the above-mentioned parent, by the above-mentioned formulas, it is seen that if the Vth3 should be equal to a Vth2 (the threshold voltage of the transistor P2) during the manufacturing process, and the amount of output current of the OLED 650 will not influenced by the voltage Vth2 (the threshold voltage of the transistor P2) and only depends on the amount of the data signal voltage (Vdata). Thus, the transistor P2 of which the variance in the threshold voltage caused by the factor of the manufacturing process can be compensated.
As described above, according to the U.S. Pat. No. 6,362,798, the requirement of the manufacturing process is higher, so as to be disadvantageous for the production yield of display panels. The patent mainly discloses that the Vth_comp must be equal to a Vth_driv during the manufacturing process, so the driving TFT M4 which of the variance in the threshold voltage caused by the factor of the manufacturing process can be compensated and the amount of output current of the OLED doesn't depend on the threshold voltage of the driving TFT M4.
Technology of the thesis published by Seoul University (Korea) is similar to that of the U.S. Pat. No. 6,362,798. According to the thesis, the requirement of the manufacturing process is also higher, so as to be disadvantageous for the production yield of display panels. The thesis mainly disclose that the Vth3 must be almost equal to the Vth2 during the manufacturing process, so the thin film transistor of which the variance in the threshold voltage caused by the factor of the manufacturing process can be compensated and the amount of output current of the OLED 650 doesn't depend on the self threshold voltage of the transistor P2.
Accordingly, there exists a need for a pixel driving circuit of an organic light emitting diode display panel to solve the above-mentioned problems and disadvantages.